1. Field of the Invention
The present invention relates to a system of transmission by modulation-demodulation of a microwave, for two-way communication between a fixed station, called a beacon or reader, and a mobile station, called a badge or responder. More precisely, the invention relates to the modem (modulator-demodulator) of the portable badge, the antenna impedance of which is matched or mismatched depending on whether the modem works, respectively, as a detector or as a responder to the signals transmitted by the fixed station. The impedance matching variations are obtained by causing variations in the biasing voltages of a transistor of the modem.
This type of data exchange can be applied to the control of moving bodies, for example in the identification of wagons on a railroad, automobiles at a toll-gate or pedestrians at the entrance to a building. The mobile station or badge takes the form of a chip card, and it has to be extremely economical in energy for it is supplied by small cells known as "buttons".
2. Description of the Prior Art
The general diagram of a system for the exchange of data by microwave electromagnetic radiation, according to the prior art, is fairly simple and FIG. 1 which represents it enables its elements and the abbreviations used to be specified.
A mobile station or badge 1 includes a microwave modem part 3, an information processing part 4 and a supply 5. The exchanges between these two parts relate to the operation of modulation/demodulation and to the transmission/reception commands.
The fixed station or beacon 2 includes a microwave source, a microwave modem part 6, an information processing part 7 and a computer 8 which enables the management of all the radioelectrical exchanges carried out with the badges, by means of an antenna 9 for each badge and an antenna 10 for each beacon or reader.
The following values will be used:
d=distance between the antennas 9 and 10, PA1 G.sub.l =gain of the antenna 10 of the reader 2 PA1 G.sub.b =gain of the antenna 9 of the badge 1 PA1 P.sub.li =power transmitted at the antenna of the reader 2 PA1 P.sub.lr =power received by the antenna of the reader PA1 P.sub.bi =power transmitted at the antenna of the batch 1 PA1 P.sub.br =power received by the antenna of the batch PA1 P.sub.br min =minimum power of triggering of the modem of the badge in the demodulation mode. PA1 P.sub.lrmin =minimum power of triggering of the demodulation chain of the reader.
The modulations used in these systems are generally of the OOK (on-off keying) type in amplitude modulation or of the PSK (phase-shift keying) type in phase modulation or of the FSK (frequency shift keying) type in frequency modulation.
The working of the modem differs, depending on whether the beacon interrogates the badge or whether the badge responds to the beacon.
In the mode in which the badge 1 is interrogated by the reader 2, the reader 2 generates (P.sub.li) a modulated microwave signal. The badge receives this signal (P.sub.br) and demodulates it: it is a step that enables the activation of the badge that has been in the vigil state.
In the mode in which the badge 1 is responding to the reader 2, the reader generates a non-modulated microwave signal. The badge receives this signal, modulates it, i.e. loads it with the information on which it has been interrogated and re-transmits a signal comprising, as the case may be, either losses or gain.
The electrical circuits used to perform the microwave function (modem) of the badge may be of different types.
The active circuits comprise the specific feature of giving power at the microwave frequencies of the carrier (of the order of 1 to 100 GHz). They therefore have gain at these frequencies, and use a biased transistor. These circuits are generally used during the retransmission of a signal from the badge towards the reader.
The semi-active circuits give no power at the microwave frequencies of the carrier, for which they have no gain, but nevertheless have gain at the demodulation frequencies of the order of 40 kHz to 1 MHz, or several MHz, but not in GHz. These circuits are valuable for increasing the detection sensitivity of the badge.
There also exist passive circuits, using diodes, switches etc. . . which have no gain, either at the microwave frequencies of the carrier (&gt;1 GHz) or at the demodulation frequencies (&lt;1 GHz). These circuits have low sensitivity to control the information processing circuit 4, at least in the case of a portable badge, since there is a problem of energy provided by the small "button" cells: these circuits therefore perform less well in detection.
The condition necessary for the transmission from the reader to the badge to be operational is: ##EQU1## with .lambda.=wavelength of the carrier wave.
As a rule, there are no power problems for the reader, since it is a fixed station that can have all the requisite energy available to it.
The condition necessary for the transmission from the badge to the reader (i.e. the response) to be operational is: ##EQU2## with G.sub.mod =loss or gain provided by the modem of the badge during the modulation of the carrier transmitted by the reader.
It has been stated that the badge has only low energy sources, and that it is appropriate to economize energy by low consumption.